The present invention relates to a method for the removal of gallium from fly ash. In many industrial processes, fly ash is produced in chimneys or stacks where electrostatic precipitators or other recovery equipment are used to remove particulates. In general, the fly ash is composed of fine grained particles having a silicate base with small amounts of some trace metals concentrated on the surfaces of the particles. Some of these trace metals are valuable, including copper, nickel, gallium, and germanium. Others, such as arsenic, lead and mercury, are toxic.
Numerous attempts have been made in the past to recover certain of the trace metals present in fly ash. Report of Investigations 6940 of the United States Department of the Interior, Bureau of Mines, entitled "Extraction of Germanium and Gallium from Coal Fly Ash and Phosphorous Furnace Flue Dust" by R. F. Waters and H. Kenworthy (1967) describes the efforts of the Bureau of Mines to recover germanium and gallium. Sublimation of these trace metals was the method described in the report. Furthermore, U.S. Pat. No. 4,475,993, issued Oct. 9, 1984, describe a process for recovering silver, gallium and other trace metals from a fine grained industrial fly ash. The process involves contacting the fly ash with aluminum trichloride in an alkali halide melt to react the trace metals with the aluminum trichloride to form compositions soluble in the melt and a residue which contains the silicate and aluminum oxide. Then, the desired trace metals are separated from the met by electrolysis or other separation techniques.
The methods described above suffer from low starting concentrations of gallium which takes them uneconomic. The method of the present invention entails a preliminary beneficiation stage in which the ash has its gallium concentration increased. Furthermore, the classification also reduces the iron content in the ash. Iron is a known contaminant in sublimation processes due to its facility for oxidizing the gallium suboxide to gallium sesquioxide. The gallium sesquioxide is a non-volatile variety of gallium which cannot be sublimed. The process of this invention is an ideal first step in gallium extraction methods.